Sensitization of silver halide emulsions with direct electrical current



United States Patent 3,411,917 SENSITIZATION OF SILVER HALIDE EMULSIONSWITH DIRECT ELECTRICAL CURRENT John Figueras, Rochester, N.Y., assignorto Eastman Kodak Company, Rochester, N.Y., a corporation of New JerseyNo Drawing. Filed Apr. 29, 1965, Ser. No. 451,992 9 Claims. (Cl. 96-107)ABSTRACT OF THE DISCLOSURE Photographic silver halide emulsions aresensitized by passing a direct electrical current having a voltage of3-12 volts therethrough. The sensitization treatment increases the speedof the photographic emulsion. It can be used in conjunction with otherchemical sensitizing treatments such as, for example, sulfur-type andnoble metal sensitization. The electrolysis treatment can be performedeither before or after the other sensitizing treatments. Especiallyuseful results have been obtained where the treatment was carried outfollowing the other sensitizing operations.

This invention relates to photography, and more particularly, to a newmethod for the reduction sensitization of photographic silver halideemulsions.

It is known that the sensitivity of silver halide emulsions can beincreased by several methods, including spectral sensitization andchemical sensitization. In spectral sensitization, the optical range ofsensitivity, is increased, while in chemical sensitization, the overallsensitivity of a photographic emulsion is improved. The presentinvention is concerned primarily with the latter type of sensitization.

Among the common forms of photographic chemical sensitizations there aresulfur-type sensitization, noble metal sensitization, and reductionsensitization. The first involves treatment of silver halide emulsionswith compounds of the sulfur family and is believed to result in theformation of sensitivity specks in or on the silver halide grain; thesecond comprises the addition of a noble metal salt or compound with thepossible formation of noble metal nuclei in or on the silver halidegrain; the third, i.e., reduction sensitization, has been performed bythe addition of compounds having a sufiiciently strong capacity toreduce silver halide to metallic silver. Reduction sensitization isextremely useful since it can be additive to the former two forms ofchemical sensitization.

It is, accordingly, an object of this invention to provide an improvedmethod of reduction sensitizing photographic silver halide emulsions.Still another object of my invention is to provide a new method ofreduction sensitization which is relatively easy to apply and whichrequires no chemical treatment. It is a further object of my inventionto provide a method of reduction sensitization which obviates thecritical features inherent in chemical procedures of reductionsensitization. Another object of this invention is to further increasethe speed of sulfur-type and/or noble metal sensitized silver halideemulsions by the reduction sensitizing method disclosed in thisinvention. It is still a further object to provide high speedreductionsensitized emulsions with low fogging propensity as well asimproved keeping stability. It is an object ofmy invention to provide animproved nonchemical method of reduction sensitization of photographicsilver halide emulsions by electrolytic means. Other objects of myinvention will appear herein.

In its broadest aspects, the aboveo'bjects are accom- 3,411,917 PatentedNov. 19, 1968 plished by passing D.C. electric current through anaqueous silver halide photographic emulsion. For example, through anaqueous silver halide photographic emulsion is passed by means of inertelectrodes a direct electrical current. I have found that either carbonor silver electrodes are eminently suitable for use in this connection.Although any voltage which will supply sufficient potential to force theelectric current through the emulsion can be used, ordinarily a valuewithin the range of 3-12 volts would be employed for an electrodespacing of 3 inches. Variation of treating conditions is possible as maybe selected by the individual operator in any particular situation. Forinstance the emulsion may be conveniently sensitized in accordance withthe invention while subjected to stirring or other agitation and at atemperature assuring good liquidity for example 40 C.

The present invention makes possible a more advantageous combination ofreduction sensitization, noble metal sensitization and sulfur-typesensitization, while maintaining, within controllable limits, fog andstability of the treated emulsions.

Where the new reduction sensitizing procedure of my invention is used inconjunction with other chemical sensitizing, for example, sulfur-typeand noble metal sensitization, the electrolysis can optionally beperformed either before or after the other sensitizing treatments.Especially useful results have been obtained where the treatment wasperformed following the other sensitizing operations.

My electrical method of reduction sensitizing photographic silver halideemulsions can be used with emulsions having a gelatin vehicle. Itoperates advantageously with emulsions containing a mixture of gelatinand one or more other hydrophilic colloids such as polyvinyl alcohol,gum arabic, ethyl acrylate-acrylic acid copolymer, butylacrylate-acrylic acid copolymer, etc.

My invention is not confined to any particular sulfurtype sensitizer orgroup of sulfur-type sensitizers, although the following areparticularly useful in practicing my invention: allylisothiourea,allylthiourea, allylisothiocyanate, phenylisothiocyanate,phenylthiourea, sodium thiosulfate, thiocarbanilide, thiourea,thiosemicarbazide, thioacetamide, thioformamide, thiobarbituric acid,etc. Such well known sensitizers have been previously described inSheppard U.S. Patent 1,574,944, issued Mar. 2, 1926; Sheppard et al.U.S. Patent 1,623,499, issued Apr. 5, 1927; Sheppard et al. U.S. Patent2,410,689, issued Nov. 5, 1946, etc.

Noble metal salts or compounds useful in sensitizing emulsions are alsowell known to those skilled in the art and comprise auric trichloride,sodium chloroaurate, potassium chloroaurite, potassium ibromaurite,potassium iodoaurite, potassium iodoaurate, potassium aurithiocyanate,potassium aurichloride, auric sulfate, sodium auric chloride, potassiumauric bromide, sodium auric bromide, alkali metal aurous thiosulfate,alkali metal aurous sulfite, thiourea-auric chloride complex, auricchloride- 3-ethyl-5-[ (3-ethyl) 2(3H)-benzoxazolyldene)ethylideneJrhodanine addition product (e.g.,Damschroder et al. U.S. Patent 2,642,361, issued June 16, 1953), etc.Typical gold sensitizers are described in Waller et al. U.S. Patent2,399,083, issued Apr. 23, 1946.

The following examples illustrate my invention and are not limitingthereof:

EXAMPLE 1 To 317 grams of optimally finished sulfur-gold sensitizedemulsion containing 0.10 mole of silver halide and 24.5 grams ofgelatin, was added 0.20 gram of 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene antifoggant in 6.0 cc. of water. The pHof the emulsion was adjusted to 6.0 and silver rod electrodes of about Ainch in diameter and 3 inches in length were immersed in the emulsion.Current was passed through the emulsion for 3 minutes by means of theelectrodes. The current source was two 6-volt dry cells wired in serieswhich provided a current from approximately 80 to 120 ma. Thiscorresponded to a current density at the electrodes of about ma./cm.After electrolysis, the emulsion pH was adjusted to 6.4 and the emulsionpAg was adjusted to 8.3. Water and additional gelatin were added and theemulsion was coated on transparent film support at a silver coverage of540 mg./ft. and a gelatin coverage of 1225 mg./ft. The emulsion coatingwas dried, sensitometrically exposed in an Eastman 1B sensitometer, anddeveloped for 5 minutes at a temperature of 68 F. in a developingsolution of the following composition:

Water ml 500 Methyl-p-aminophenol gm 2.5 Sodium sulfite, des. 'gm 30.0Hydroquinone m 2.5 Sodium metaborate, 8H O gm 10.0 Potassium bromide gm0.5 Water to make liter 1.0

A similar coating was made of unelectrolyzed emulsion.

After development, the film samples were fixed, Washed, and dried in theconventional manner.

The sensitometric results were as follows:

Gamma Rel. Speed Fog Nonelectrolyzed coating. 100 2. 0. 04

Electrolyzed coating 182 2. l0 0. 08

EXAMPLE 2 Y A fine grain negative silver bromoiodide gelatin emult Rel.Speed Fog Nonelectrolyzed coating 100 0. 10 Electrolyzed coating 145 0.23

The electrolysis increased the fog level of this particular emulsionused; but after allowance is made for this fog, it is seen thatelectrolysis has increased the emulsion speed by approximately That theoccurrence of fog is not a necessary feature of the electrolytic processof my invention is shown in the following example.

EXAMPLE 3 This table shows that azaindene antifoggants keep the fog fromincreasing when they are present during reduction sensitizing byelectrolysis.

Similar results are obtained with other types of azaindenes such asthose described in Heimbach and Kelly U.S. Patents 2,444,605, and2,444,606, issued July 6, 1948; Heimbach U.S. Patents 2,444,607, issuedJuly 6, 1948, and 2,450,397, issued Sept. 28, 1948; Heimbach and ClarkU.S. Patent 2,716,062, issued Aug. 23, 1955; Allen and Reynolds U.S.Patents 2,713,541, issued July 19, 1 955, and 2,743,181, issued Apr. 24,1956; Carroll and Beach U.S. Patent 2,716,062, issued Aug. 23,1955;Allen and Beilfuss U.S. Patent 2,735,769, issued Feb. 21, 1956; Reynoldsand Saga] U.S. Patent 2,756,147, issued July 24, 1956; Allen and SaguraU.S. Patent 2,772,164, issued Nov. 27, 1956, and those disclosed by Birrin Z. wiss Phot., Vol. 47, 1952, pages 2-28.

The new method of reduction sensitization of my invention is alsoapplicable in the presence within an emulsion of other known stabilizingcompounds such as the triazoles of Heimbach and Kelly U.S. Patent2,444,608, issued July 6, 1948; the zinc and cadmium salts of Jones U.S.Patent 2,839,405, issued June 17, 1958; the carboxymethylmercaptocompounds of Murray, Reynolds and Van Allan U.S. Patent 2,819,965,issued Jan. 14, 1958; and the disulfides of Kodak Belgian Patent569,317, issued July 31, 1958.

Examples 4, 5 and 6 show respectively the electrolysis effect onemulsions which (a) bear no prior chemical sensitization, (b) are sulfursensitized, and (c) are sulfurgold sensitized.

EXAMPLE 4 Medium grain silver bromoiodide gelatin emulsion containing 2gm./silver mole of 4-hydroxy-6-methyl-1, 3,3a,7-tetraazaindene buthaving no chemical sensitization was subjected to a direct currentelectrolysis as described in Example 1. The photographic productobtained was tested in the manner previously described Sulfur sensitizedcoarse-grain silver bromoiodide gelatin emulsion containing no4-hy-droxy-6-methyl-1,3,3a,7- tetraazaindene was employed as theemulsion in a run similar to that of Example 1. The following resultswere obtained:

The same type of emulsion as described under Example Rel. Speed Fog 2was electrolyzed 1n the presence of 2.0 gm./ silver mole NonelectrolyzedSample 100 M6 of 4-hydroxy-6-methyl-1,3,3a,7-tetraaza1ndene antifoggantElectrolyzed sample 132 0.08 and compared with an emulsion sampleelectrolyzed without containing the above addendum.

EXAMPLE 6 3g} Fog Sulfur-gold and selenium-gold sensitized medium-grainS 1 h dr 6 th 1133 7 silver bromoiodide gelatin emulsions containing 2gm./ 1 B,2,;; g; j Xjffiffi j M3 5 silver mole of4-hydroxy-6-methyl-1,3,3a,7-tetraazain- Strang e v viitgg ggn gs gvgr 11y o y-fino 0 12 6 dene were electrolyzed as described in Example 1. The

m I z m em results obtained were as follows:

Non-Electrolyzed Eleotrolyzed Speed Gamma Fog Speed Gamma FogSulfur-gold sensitization 100 2.64 0.06 2.56 0.08 Selenium-goldsensitization 100 2.60 0.20 162 2.60 0.22

Photographic speed increasing compounds can be used in emulsions whichare reduction sensitized by the electrolytic method of my invention.Such compounds can be of the quaternary ammonium type of Carroll US.Patent 2,271,623, issued Feb. 3, 1942; Carroll and Allen US. Patent2,288,226, issued June 30, 1942; and Carroll and Spence US. Patent2,334,864, issued Nov. 23, 1943; or the polyethylene glycol type ofCarroll and Beach US. Patent 2,708,162, issued May 10, 1955; or thequaternary ammonium salts and polyethylene glycols of Piper US. Patent2,886,437, issued May 12, 1959; as well as the thiopolymers of Grahamand Sagal US. Patent 3,046,129, issued July 24, 1962, and Dann andChechak US. Patent 3,046,134, issued July 24, 1962.

In experiments performed with emulsions containing speed increasingcompounds mentioned above, significant increases in speed were shown bythe electrolyzed emulsion over the control, and the speed effects of theaddenda were approximately additive to the speed gained fromelectrolysis.

The electrolytic procedure of my invention is also applicable when ahigher degree of reduction even up to complete fogging of a silverhalide emulsion is desired, as for example in the preparation ofphotographic direct positive materials. The reduction of silver halidegrains for the preparation of direct positive materials is commonlyachieved either by the addition to a photographic negative emulsion ofcompounds such as formaldehyde (see Kendall and Hill, US. Patent2,541,472) or stannous chloride or by exposing the emulsion to highintensity light radiation.

Accordingly, my invention also provides a new method for reductionfogging of photographic emulsions for use as direct positive materials.This method consists generally of subjecting an emulsion to electrolysisas described above, in which electrolytic potential, time of treatment,and other conditions are selected in order to achieve the degree ofreduction desired. The practice of my invention envisions thecombination of conventional treatment by chemical addenda withelectrolytic reduction to achieve the degree of reduction needed in aparticular circumstance.

In the practice of my invention, electrical potentials ranging from 3 to12 volts have been applied. Higher or lower potentials can be useddepending on the internal resistance and the current flow desired.Potentials ranging from approximately 1 /2 volts to at least 12 voltsare considered useful, depending on the operating conditions and desiredextent of emulsion reduction.

The speed increase from electrolysis appears to be the result of animprovement in granularity efiiciency rather than a development efiect.This is shown by two pieces of evidence. First, the sensitometric speedgain from electrolysis is independent of the degree of development.Secondly, the speed obtained from electrolysis is not accompanied by anyincrease in the granularity of the film.

The invention has been described in considerable detail with particularreference to certain preferred embodiments thereof, but it Will beunderstood that variations and modifications can be effected within thespirit and scope of the invention as described hereinabove, and asdefined in the appended claims.

I claim:

1. A method of sensitizing an aqueous silver halide photographicemulsion which comprises passing a direct electrical current having avoltage of 3-12 volts therethrough.

2. A method of sensitizing a sulfur-sensitized aqueous silver halidephotographic emulsion which comprises passing a direct electricalcurrent having a voltage of 3-12 volts therethrough.

3. A method of sensitizing a noble-metal sensitized aqueous silverhalide photographic emulsion which comprises passing a direct electricalcurrent having a voltage of 3-12 volts therethrough.

4. A method of sensitizing an aqueous silver halide photographicemulsion, which has been chemically sensitized with a labile element ofthe sulfur family and a noble metal compound, which comprises passing adirect electrical current having a voltage of 3-12 volts therethrough.

5. A method of sensitizing a selenium sensitized aqueous silver halidephotographic emulsion which comprises passing a direct electricalcurrent having a voltage of 3-12 volts therethrough.

6. A method of sensitizing an aqueous silver halide photographicemulsion containing an antifoggant which comprises passing a directelectrical current having a voltage of 3-12 volts therethrough.

7. A method of sensitizing an aqueous silver halide photographicemulsion which comprises passing therethrough a direct electricalcurrent having a voltage of 3- 12 volts using silver electrodes.

8. A method of sensitizing an aqueous silver halide photographicemulsion which comprises passing therethrough a direct electricalcurrent having a voltage of 3-12 volts using carbon electrodes.

9. A method of producing a fogged aqueous silver halide photographicemulsion useful in the production of photographic direct positivematerials which comprises passing a direct electrical current having avoltage of 3-12 volts through an aqueous silver halide photographicemulsion until said fogged aqueous silver halide photographic emulsionis obtained.

References Cited UNITED STATES PATENTS 1,602,592 10/1926 Sheppard 961071,962,133 6/1934 Brooker et a1. 96-109 3,148,276 9/1964 Rothstein 250-3,189,458 6/1965 Herz 96-108 NORMAN G. TORCHIN, Primary Examiner.

J. R. EVERETT, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,411,917 November 19, 1968 John Figueras It is certified that errorappears in the above identified patent and that said Letters Patent arehereby corrected as shown below:

Column 1 line 31, after "sensitivity" cancel the comma. Column 2, line58, "-2- (3H)" should read 2 (3H) Column 4, line 9, "2,716,062, issuedAug. 23, 1955" should read 2,444,609, issued July 6, 1948 Signed andsealed this 10th day of March 1970.

(SEAL) Atlest:

Edward M. Fletcher, Jr. WILLIAM E.

Attesting Officer Commissioner of Patents

